In a phospholipid bilayer constituting a cell membrane, there are a large number of proteins, lipids other than phospholipids, and the like, some of which function as growth factor receptors, cell adhesion factors, ion channels and the like. The molecules located on the cell membrane can move relatively freely in the cell membrane, and assemble and dissociate repeatedly. Particularly, a structure called raft, which is formed by an assembly of a plurality of molecules located on the cell membrane and is exposed on the surface of the cell membrane, plays an important role, for example, as a receptor for bacterium, virus or the like, or as a platform for transmitting extracellular information into the cell. Accordingly, it is very important in biochemical research to know with what kind of other molecules located on the cell membrane, a certain molecule located on the cell membrane functions in coordination.
However, the analysis of interaction between molecules located on the cell membrane is very difficult. For example, an immunoprecipitation method is known as a method for separating and detecting a protein interacting with a certain target protein, in which the target protein is selectively precipitated by acting an antibody specific for the target protein with the target protein. However, in the method, it is difficult to reflect the interacted state of the target protein and the other protein under a physiological condition. This is because, in the method even if the other protein interacts with the target protein located on the cell membrane, such interaction may possibly be cancelled at the stage where the target protein is separated from the cell membrane. Moreover, although no interaction between the proteins occurred on the cell membrane of a living cell, a pseudo-interaction may occur at the stage of separating the proteins.
As another method, a cross-linker method is known, in which a protein interacting with a target protein is identified by coupling a cross-linker with the target protein and hereafter cross-linking the interacting protein. In the method, however, the length and the shape of the cross-linker molecule are fixed; therefore, only the closely contacted protein can be cross-linked and detected. As a result, there are few examples in which the interaction between molecules on the cell membrane could be successfully analyzed by the cross-linker method.
Accordingly, specialized techniques for detection of the interaction between molecules on the cell membrane, other than the methods described above, were examined.
For example, Patent Document 1 describes a screening method of a substance interacting with an ABC (ATP Binding Cassette) protein located on a cell membrane. In the method, a membrane fraction where the ABC protein has been expressed, a labeled nucleoside triphosphate, a nucleoside diphosphate-immobilized substance and the test substance, are contacted together.
Patent Document 2 describes a method of screening a candidate compound interacting with transmembrane proteins, by expressing the transmembrane proteins on a cell and then contacting the candidate compound therewith to detect the change in the distribution of the transmembrane proteins compared with the situation where the candidate compound was not contacted with the transmembrane proteins.    Patent Document 1: JP2005-24245A    Patent Document 2: JP2005-522227T